Nioh and Nioh 2 (including all DLCs) are also bundled in the Nioh Collection on PS5.
Shisenin Kosen is an optional dojo boss in Nioh. You will encounter the boss in the Dojo Mission Zen and the Sword are One, which is a mission that you’ll unlock if you have a 500,000 proficiency with the Katana. You must also complete the Main Mission Immortal Flame to unlock this Dojo Mission.
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Yagyu Sekishusai Boss Guide
Both of you will only be able to use a katana in the battle. Since he will attack you immediately once you approach him, block his attacks as a dodge can be quite difficult to do during the situation. Bash him with the katana repeatedly then run away from him again. Once he does his katana slice that makes him move forward, dodge to the side then attack. Repeat this process until he runs out of Ki, then you will be able to do finishers on him.
He moves quite fast, so you have to either dodge his attacks if you can or block it when he immediately strikes you. Dodging and going behind him is an easy way to get him open for your combo.
Defeating him unlocks the mystic art for the sword and a Sword Master Trophy.